This invention relates to an improved method for injection molding nitrile articles. More particularly, the present invention is directed to the injection molding of articles made of high nitrile polymers wherein said polymer is first charged to a vacuum hopper system for application of a vacuum from which it is fed into a reciprocating screw injection molding machine wherein the article is formed.
While the injection molding of plastic materials is generally well known and has grown with the increased use and new application found for such products, it is a complex technology with many overlapping and interrelating facets. High nitrile polymers possess a number of chemical and physical properties which make them attractive for use in a variety of packaging application, particularly molded containers such as bottles, cans and jars. However, such materials generally have relatively high melt viscosities and therefore are not particularly well suited for injection molding. By careful control of conditions, such polymers can be injection molded, however, the thermal sensitivity of nitrile polymers make it extremely difficult to attain and control the desired optical properties, i.e. color, clarity and haze.
The use of vacuum hoppers in extrusion processes has been known for a number of years as disclosed in "Vacuum Hopper Extrusion" by N. T. Flathers et al, Modern Plastics, v. 37, p. 105, Apr. 1960 and "Advances in Dry Blend Extrusion" by N. T. Flathers et al, Modern Plastics, v. 38, p. 210, May 1961. Such techniques have not been considered as popular or efficient as venting techniques and do not appear to be attractive to injection molding operations and particularly those involving reciprocating screw injection molding machines because of the difficulty in adapting and maintaining such a system.